1. Field of the Invention
The present invention relates to photoreceptor drums, and a developing cartridge and an image forming apparatus having the photoreceptor drums, and more particularly, to photoreceptor drums in which assembling tolerances are improved to maintain a uniform developing gap or a uniform developing nip, and a developing cartridge and an image forming apparatus having the photoreceptor drums.
2. Related Art
Electrophotographic image forming apparatuses, such as printers, photocopiers, facsimile machines and multi-functional products, are devices for printing an image on a recording medium, such as a paper, by an electrophotographic process. In such an image forming apparatus, an electrostatic latent image is formed on a photosensitive body, such as photoreceptor drum, and developed into a toner image by a developing roller. typically, a developing cartridge, which includes a developing roller and toner, is removably mounted in the main body of the image forming apparatus. The photoreceptor drum is either mounted in the developing cartridge or fixed to the main body of the image forming apparatus, according to the type of the image forming apparatus utilized.
Developing methods can be divided into two categories according to whether the developing roller contacts the photoreceptor drum. When the developing roller does contact the photoreceptor drum, a developing nip is formed, and a developing action is performed in the developing nip. When the developing roller does not contact the photoreceptor drum, a developing gap is formed, and a developing action is performed in the developing gap. To ensure satisfactory toner image development, the developing roller and the photoreceptor drum must be maintained in parallel and at a constant distance. Otherwise, the uniformity of the developing nip and the developing gap can be reduced, thereby degrading the image quality. If the developing roller and the photoreceptor drum are not correctly aligned, tone image failure may occur on the photoreceptor drum. That is, an image may be uneven or contain unprinted voids.
To avoid the above problems, the developing nip and the developing gap must be uniformly maintained in an axial direction of the photoreceptor drum and the developing roller. However, fine control of the developing nip and the developing gap is very difficult. The assembly tolerance of a plurality of parts for assembling a driving unit of the photoreceptor drum and a position determining unit must be strictly managed. When a pair of shafts support the ends of the photoreceptor drum, the shafts are must be axially aligned. A photoreceptor drum fixing device having two step shafts that support a photoreceptor drum has been disclosed, for example, in U.S. Pat. No. 5,749,028 entitled “Multi-size Photoreceptor Flange Bearing” and U.S. Pat. No. 5,444,546 entitled “Photoreceptor Drum Axle Improvement”.
The tolerance management of the shafts and the photoreceptor drums are particularly important. This is because the shaft rotates in contact with the photoreceptor drum. As the contact area between the shaft and the photoreceptor drum increases, the management of the manufacture and assembly tolerances becomes increasingly difficult. However, if the contact area is too small, the shaking of the shaft with respect to the sleeves of the photoreceptor drum may occur, and the photoreceptor drum may not stay in parallel to the developing roller.
To print a color image, four (4) developing cartridges including four different color toners, such as cyan “C”, magenta “M”, yellow “Y”, and black “K”, must be included. The number of photoreceptor drums may be increased from one (1) to four (4) according to the printing method utilized. In an image forming apparatus that prints a color image, the number of developing rollers and photoreceptor drums increases, thereby increasing the difficulty of maintaining a uniform developing nip and developing gap.